The present invention relates in general to communication systems, and is particularly directed to a hybrid RF power amplifier linearization mechanism that provides very high output distortion rejection and enhanced amplifier linearity. This hybrid architecture includes a main RF amplifier stage coupled with a carrier cancellation loop, the extracted residual distortion output of which feeds a feed-forward loop containing an auxiliary error amplifier stage. The main RF power amplifier stage is configured of a pair of parallel RF power amplifiers installed in intermod-complementing predistortion paths of the type disclosed in the U.S. Patent to Mucenieks, No. 6,111,462 (hereinafter referred to the ""462 patent and the disclosure of which is incorporated herein). The outputs of the main and auxiliary amplifier stages are combined to produce a composite amplified RF output signal having very substantially reduced intermodulation products.
Communication service providers are subject to very strict bandwidth usage spectrum constraints, including technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC). These rules require that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 50 dB). Although these regulations may be easily met for traditional forms of modulation, such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation.
Attenuating the sidebands sufficiently to meet industry and regulatory-based requirements by such modulation techniques requires very linear signal processing systems and components. Although linear components can be produced at a reasonable cost at the relatively narrow bandwidths (baseband) of telephone networks, linearizing inherently non-linear components such as RF power amplifiers can be prohibitively expensive.
A fundamental difficulty in linearizing RF power amplifiers is the fact that they generate unwanted intermodulation distortion products (IMDs) which manifest themselves as spurious signals in the amplified RF output signal, such as spectral regrowth or spreading of a compact spectrum into spectral regions that do not appear in the RF input signal. This distortion causes the phase/amplitude of the amplified output signal to depart from the phase/amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the RF input signal.
An inefficient approach to linearizing an RF power amplifier is to build the amplifier as a large, high power device, and then operate the amplifier at a low power level (namely, at only a small percentage of its rated output power), where the RF amplifier""s transfer characteristic is relatively linear. An obvious drawback to this approach is the overkill penaltyxe2x80x94a costly and large sized RF device.
Other prior art linearization techniques include baseband polar (or Cartesian) feedback, post-amplification, feed-forward correction, and pre-amplification, pre-distortion correction. In the first approach, the output of the RF power amplifier is compared to the input, and a baseband error signal is used to directly modulate the signal which enters the amplifier. In the second approach, error (distortion) present in the RF amplifier""s output signal is extracted, amplified to the proper level, and then reinjected (as a complement of the error signal back) into the output path of the amplifier, so that (ideally) the RF amplifier""s distortion is effectively canceled.
Pursuant to a third approach, a predistortion signal is injected into the RF input signal path upstream of the RF amplifier. Ideally, the predistortion signal has a characteristic equal and opposite to the distortion expected at the output of the RF amplifier. As a result, when subjected to the (distorting) transfer characteristic of the RF amplifier, it effectively cancels the distortion in the output. Predistortion may be made adaptive by measuring the distortion at the output of the RF amplifier and adjusting the predistortion control signal to minimize the distortion of the output signal of the power amplifier during real time operation.
In accordance with the invention described in the above-referenced ""462 Patent, and diagrammatically illustrated in FIG. 1, high efficiency RF power amplifier linearization is achieved by coupling a pair of effectively matched RF power amplifiers A1 and A2 in circuit with one another in a manner that causes one RF power amplifier to xe2x80x98pre-distortxe2x80x99 the other. For purposes of the present discussion, this parallel configured, effectively matched amplifier predistortion architecture will be referred to as an active cancellation technique (ACT) amplifier architecture. Being effectively matched implies that the two RF power amplifiers A1, A2 have essentially the same transfer characteristicsxe2x80x94both in terms of their intended RF performance and unwanted IMD components they inherently introduce into their amplified outputs.
As shown in FIG. 1, an RF input signal to be amplified is split by a directional coupler CPL1 into two paths. A first path includes an attenuator or scaling pad ATT and a controlled gain adjustment G1, which serve to adjust the RF input signal in amplitude prior to being amplified by the main amplifier A1. The output of the main path amplifier A1 is coupled through a delay stage DL2 to a first input of an output combining stage OCS (such as a quadrature hybrid).
A second split RF input signal path is used to construct a signal containing both of the original RF input signal to be amplified by the second amplifier A2, and a complementary version of the IMD products each of the two amplifiers inherently introduces. Distortion is extracted using carrier cancellation combiner circuitry very similar to that found in most conventional feed-forward RF power amplifiers. The extracted distortion products are adjusted in amplitude and phase and combined with an appropriately delayed sample of the RF input signal.
For this purpose, the second path from the directional coupler CPL1 is coupled though a delay stage DL1 to a first input of a Wilkinson splitter WS1, a first output of which is coupled to a first Wilkinson combiner WC1. A second output of Wilkinson splitter WS1 is coupled through a variable gain stage G2 to a first input of a second Wilkinson combiner WC2, a second input of which is coupled to the output of the first Wilkinson combiner WC1. A second input of the first Wilkinson combiner WC1 is coupled through a variable phase adjustor "PHgr"1 to a directional coupler CPL2 installed in the output path of the main path amplifier A1.
The output of the second Wilkinson combiner WC2, which is a composite of the RF input signal and distortion products extracted from the first or main matched RF amplifier A1, is coupled through a variable gain stage G3 and variable phase adjustor "PHgr"2 to the second matched or error RF amplifier A2. Namely, the error amplifier A2 is driven by the distortion products extracted from the main amplifier A1. The output of the second matched RF amplifier A2 is coupled to a second input of the output combining stage OCS.
The amplitude of the RF input signal component of the composite RF signal driving the error amplifier A2 is adjusted to be the same as the amplitude of the pure RF input signal driving the main amplifier A1. Namely, the phase and amplitude of the distortion products are adjusted so that they not only cancel the distortion products generated by the input signals applied to the error amplifier A2, but also replace these distortion products with equal amplitude anti-phase replicas of these products. Thus, the delayed output of the main amplifier A1 and the undelayed output of the matched error amplifier A2 contain equal phase and amplitude amplified RF input signals and equal amplitude anti-phase distortion products. As a consequence, distortion components resulting from the RF input signal components driving both the main and error amplifiers are essentially the same.
In the output combining stage OCS, these signals are summed, so that the (desired) amplified RF signals add and the (unwanted) distortion products cancel. The output from the combining stage OCS is therefore an amplified version of the RF input signal, that is substantially free of distortion, even though both the main and error amplifiers contain distortion products at their outputs. Both the main and error amplifiers contribute essentially equal amounts of amplified signal power to the output of the overall amplifier system. Operating efficiency is better than that of a conventional feed forward amplifier because the entirety of the error amplifier output power appears at the output of the combining stage.
It should be noted that the ACT architecture of FIG. 1 is not a conventional feed-forward amplifier. Rather, it is a very effective type of RF pre-distortion amplifier structure, in which the source of the energy used to pre-distort the error amplifier is produced by an identical (main) amplifier that is driven by essentially the same input signals as the error amplifier. The level of the distortion components of the energy driving the error amplifier is approximately 30 dB below the RF input signal component. Thus, the dynamics of both amplifiers is controlled by the dominant input signal energy.
Now although the ACT amplifier-based linearization scheme described in the ""462 Patent architecture is very effective in achieving non-linear distortion rejection at least on the order of 50 dB and greater, third generation (3G) time division multiple access (TDMA) multicarrier RF power amplifiers must attain very high distortion rejection levels (e.g., on the order of xe2x88x9265 to xe2x88x9275 dBc) which are not achievable in the ACT architecture.
Pursuant to the invention, this increased distortion rejection requirement is realized by a hybrid combination of two distinct linearization techniquesxe2x80x94an ACT amplifier stage, and an associated a feed-forward amplifier (FF) stage. Compared to a classical feed-forward design, the hybrid scheme of the present invention is capable of achieving very large IMD suppression due to the use of an ACT power amplifier stage rather than a conventional class AB power amplifier.
In accordance with a first embodiment of the invention, a main amplifier stage, to which a multicarrier RF signal is applied, is configured as an ACT power amplifier stage of the type disclosed in the ""462 Patent. A main RF signal flow path to the ACT power amplifier stage includes a processor-controlled vector modulator containing a variable gain stage and a processor-controlled variable phase shifting stage. The ACT power amplifier stage is embedded in a multicarrier cancellation loop that estimates the residual distortion in the output of the amplifier. Multitone pilot signals are injected into the amplifier""s output signal path and are used in conjunction with a pilot tone receiver to control amplitude and phase alignment of signals in the feed-forward loop.
A portion of the combined pilot tone and amplified output signal is coupled to a carrier cancellation combiner of the first loop. A second input of the carrier cancellation combiner is coupled through a delay line to the RF input port. A portion of the RF input signal delayed through the first delay line is also applied to a correlator. The carrier cancellation combiner provides an estimate of the residual distortion produced by the ACT power amplifier.
Carrier rejection in the first loop is optimized by a digital signal processor (DSP)-controlled regulator that computes the correlation between the output signal from the carrier cancellation combiner and the delayed RF input signal. The DSP processes the output signals from the correlator to derive control signals for setting the parameters of the variable attenuator and phase shifter of the vector modulator in the input path to the ACT power amplifier. The output of the carrier cancellation combiner is coupled to a second (feed-forward) signal processing loop (Loop 2), which contains a variable gain stage and a variable phase shifter of a vector modulator installed in the input path of a feed-forward error amplifier. The DSP also controls the operation of the ACT amplifier, as described in the ""462 patent.
Pilot tones detected by a pilot tone receiver are coupled to the digital signal processor which measures the degree of amplitude and phase match of the detected pilot signals, and drives the variable attenuator and the phase shifter in the input path to the loop 2 error amplifier to reduce distortion. The loop 2 feed-forward error amplifier amplifies gain and phased adjusted residual power amplifier distortion and applies it to an output combiner. The output combiner also receives a delayed pilot tone-injected output of the ACT power amplifier. The output combiner recombines the amplified feed-forward signal anti-phase with a delayed version of the output signal from main path ACT power amplifier stage, to realize very high distortion suppression.
In a second embodiment of the invention, the ACT power amplifier stage is replaced by a modified ACT power amplifier stage, in which the variable attenuator and phase adjustor components in the input path to the feed-forward RF amplifier of the ACT stage are replaced by a controlled predistortion (or PreD) unit, such as that described in U.S. Pat. No. 6,104,241 (hereinafter referred to as the ""241 Patent) to A. Cova et al, assigned to the assignee of the present application and the disclosure of which is incorporated herein. This augmented linearity (high efficiency feed-forward RF amplifier with predistortion enhancement) main path amplifier rejects higher levels of output intermodulation distortion (IMD), does not require accurate matching of the main and auxiliary amplifiers (A1, A2) and operates essentially as the first embodiment.
Pursuant to a third embodiment of the invention, the correlator of the first embodiment is replaced by a power detector. The DSP executes error minimization algorithms using the output of the power detector to generate signals for controlling the variable attenuator and phase shifter components in the input path to ACT power amplifier stage.
In a fourth embodiment of the invention, the pilot tone generator and receiver components for controlling loop 2 are removed. Instead, loop 2 is controlled by the DSP, using signals from an output distortion detector (performance monitor) coupled to the output combiner, so as to adaptively control the variable attenuator and the phase shifter in the feed-forward loop to minimize the output distortion. The fourth embodiment offers improved amplifier linearity due to the fact that the distortion detector continuously monitors the distortion at the output of the amplifier and uses this information to optimize distortion cancellation in loop 2.
A fifth embodiment of the invention integrates features of the first four embodiments to realize specific performance, complexity and cost objectives, such as the use of a loop 1 DSP controller, ACT+PreD amplifier stage, and distortion detector based control of loop 2. The fifth embodiment employs a distortion detector in place of the pilot tone generator and receiver components for controlling loop 2 and incorporates the ACT+PreD power amplifier of the second embodiment, as well as the detected power minimization scheme of the third embodiment in place of the correlator controller.